Stability of the Zwitterionic form of 4-aminopyrimidine HCl and -HBr Complexes: Computational Study

ABSTRACT. We present calculations for the 4-aminopyrimidine (4AP) – HCl and –HBr complexes. We pre-dict that the charge separated (zwitterionic) form [4APH + – Cl ‒ ] is not stable, but that [4APH + – Br ‒ ] is stable enough for experimental detection in gas phase at low temperatures. The latter observation is attributed to smaller dissociation energy of HBr compared with HCl, and to “solvation” of HBr by the amino group in 4AP. Keywords: Aminopyrimidine – acid complex, Intramolecular solvation INTRODUCTION Proton transfer from acid to base is an important process in many fields of chemistry. The tendency to accept or donate proton is a key property of molecules, especially in biochemical and physiolo-gical processes. The acidity and basicity are usually measured in the solution phase, whose strengths depend on the intrinsic structure of the substance as well as on the interactions with solvent. Recent development of gas phase experi-mental technique and quantum chemical methods now allow systematic investigation for elucidating the macroscopically measured acidity and basicity in terms of the molecular structure. The influence of solvent may also be effectively examined by studying the properties of microsolvated molecules.